U.S. Pat. No. 4,101,617 to Friederich discloses a method for producing plastic molded parts that have hollow sections using the so-called internal gas pressure method. An injection molding tool has a cavity, the walls of which define the surface of the part being produced. Plastic melt is injected into the closed tool through a plastic injection nozzle. Before the plastic melt hardens, a pressurized fluid, typically pressurized nitrogen gas, is injected into the interior of the plastic melt, so that the melt is pressed against the cavity walls thus forming hollow spaces. The gas pressure is maintained until the molded part cools down. The method advantageously achieves the result that the molded part is lighter, requires less plastic to produce, and has especially good surface characteristics, since the contraction of the material due to cooling is counteracted by the internal gas pressure. This effect prevents the formation of sink marks on the surface of the molded part.
The Friederich patent treats the simple case where the molded part being produced is essentially a tube that is to be formed as a hollow part by the internal gas pressure method. By analogy, the method can also be used to produce more complex molded parts having hollow regions as well as regions that are solid plastic.